What initiates speciation in passion-vine butterflies?

Studies of the continuum between geographic races and species provide the clearest insights into the causes of speciation. Here we report on mate choice and hybrid viability experiments in a pair of warningly colored butterflies, Heliconius erato and Heliconius himera, that maintain their genetic integrity in the face of hybridization. Hybrid sterility and inviability have been unimportant in the early stages of speciation of these two Heliconius. We find no evidence of reduced fecundity, egg hatch, or larval survival nor increases in developmental time in three generations of hybrid crosses. Instead, speciation in this pair appears to have been catalyzed by the association of strong mating preferences with divergence in warning coloration and ecology. In mate choice experiments, matings between the two species are a tenth as likely as matings within species. F1 hybrids of both sexes mate frequently with both pure forms. However, male F1 progeny from crosses between H. himera mothers and H. erato fathers have somewhat reduced mating success. The strong barrier to gene flow provided by divergence in mate preference is probably enhanced by frequency-dependent predation against hybrids similar to the type known to occur across interracial hybrid zones of H. erato. In addition, the transition between this pair falls at the boundary between wet and dry forest, and rare hybrids may also be selected against because they are poorly adapted to either biotope. These results add to a growing body of evidence that challenge the importance of genomic incompatibilities in the earliest stages of speciation.

The phylogeny of closely related species as revealed by the genealogy of a speciation gene, Odysseus

Molecular differentiation between races or closely related species is often incongruent with the reproductive divergence of the taxa of interest. Shared ancient polymorphism and/or introgression during secondary contact may be responsible for the incongruence. At loci contributing to speciation, these two complications should be minimized (1, 2); hence, their variation may more faithfully reflect the history of the species' reproductive differentiation. In this study, we analyzed DNA polymorphism at the Odysseus (OdsH) locus of hybrid sterility between Drosophila mauritiana and Drosophila simulans and were able to verify such a prediction. Interestingly, DNA variation only a short distance away (1.8 kb) appears not to be influenced by the forces that shape the recent evolution of the OdsH coding region. This locus thus may represent a test case of inferring phylogeny of very closely related species.

Incipient speciation by sexual isolation in Drosophila: Concurrent evolution at multiple loci

Drosophila melanogaster from Zimbabwe and nearby regions shows strong but asymmetric sexual isolation from its cosmopolitan counterparts. By creating stable chromosome-substitution lines, earlier studies were able to show that the two major autosomes have very large effects on both male mating success and female mating preference. In this study, we genetically dissect this sexual isolation by recombination analysis between a whole-chromosome substitution line (which carries a Zimbabwe-derived third chromosome) and a strain with seven visible markers on that chromosome. Four loci are responsible for male mating success and three others are found to control female mating preference. Because male and female traits are not closely linked, their strong association among isofemale lines is most likely a reflection of sexual selection in nature. The results suggest that a large number of behavioral loci may evolve concurrently in the incipient stage of speciation before other aspects of reproductive isolation (such as hybrid sterility) have become evident. The results shed light on the population genetic processes underlying the formation of nascent species, as well as modes of speciation.

Induction of male sterility in plants by metabolic engineering of the carbohydrate supply

Extracellular invertase mediates phloem unloading via an apoplastic pathway. The gene encoding isoenzyme Nin88 from tobacco was cloned and shown to be characterized by a specific spatial and temporal expression pattern. Tissue-specific antisense repression of Nin88 under control of the corresponding promoter in tobacco results in a block during early stages of pollen development, thus, causing male sterility. This result demonstrates a critical role of extracellular invertase in pollen development and strongly supports the essential function of extracellular sucrose cleavage for supplying carbohydrates to sink tissues via the apoplast. The specific interference with phloem unloading, the sugar status, and metabolic signaling during pollen formation will be a potentially valuable approach to induce male sterility in various crop species for hybrid seed production.